Which Kings Used Royal Taste Testers ?o?o?o?o?o?

I don't know which kings used royal taste testers, but I do know that there was once a king who lived at the edge of the Roman Empire and his name was Mithradates. He was paranoid that the Romans would one day poison him and take over his empire, so each day he'd take a little dosage of different kinds of poisons. Well, when the Romans finally did come, he tried to kill himself through poisoning but it didn't work because he'd built up a resistance!